docs: def Guzhov(x, rhol, rhog, m, D):
      r'''Calculates void fraction in two-phase flow according to the model
      in [1]_ as shown in [2]_ and [3]_.
  
      .. math::
          \alpha = 0.81[1 - \exp(-2.2\sqrt{Fr_{tp}})]\alpha_h
          
          Fr_{tp} = \frac{G_{tp}^2}{gD\rho_{tp}^2}
          
          \rho_{tp} = \left(\frac{1-x}{\rho_l} + \frac{x}{\rho_g}\right)^{-1}
          
      Parameters
      ----------
      x : float
          Quality at the specific tube interval []
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
      m : float
          Mass flow rate of both phases, [kg/s]
      D : float
          Diameter of the channel, [m]
  
      Returns
      -------
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
  
      Notes
      -----
  
      Examples
      --------
      >>> Guzhov(.4, 800, 2.5, 1, .3)
      0.7626030108534588
  
      References
      ----------
      .. [1] Guzhov, A. I, Vasiliĭ Andreevich Mamaev, and G. E Odisharii︠a︡. A 
         Study of Transportation in Gas-Liquid Systems. Une Étude Sur Le 
         Transport Des Systèmes Gaz-Liquides. Bruxelles: International Gas Union,
         1967.
      .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
         Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
         1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
         921-27. doi:10.1016/j.icheatmasstransfer.2008.04.001.
      .. [3] Woldesemayat, Melkamu A., and Afshin J. Ghajar. "Comparison of Void 
         Fraction Correlations for Different Flow Patterns in Horizontal and 
         Upward Inclined Pipes." International Journal of Multiphase Flow 33, 
         no. 4 (April 2007): 347-370. doi:10.1016/j.ijmultiphaseflow.2006.09.004.
      '''
      rho_tp = ((1-x)/rhol + x/rhog)**-1
      G = m/(pi/4*D**2)
      V_tp = G/rho_tp
      Fr = Froude(V=V_tp, L=D, squared=True) # squaring in undone later; Fr**0.5
      alpha_h = homogeneous(x, rhol, rhog)
      return 0.81*(1 - exp(-2.2*Fr**0.5))*alpha_h